(from Greek diapeirein, "to pierce"), geological structure consisting of mobile material that was forced into more brittle surrounding rocks, usually by the upward flow of material from a parent stratum. The flow may be produced by gravitational forces (heavy rocks causing underlying lighter rocks to rise), tectonic forces (mobile rocks being squeezed through less mobile rocks by lateral stress), or a combination of both. Diapirs may take the shape of domes, waves, mushrooms, teardrops, or dikes. Because salt flows quite readily, diapirs are often associated with salt domes or salt anticlines; in some cases the diapiric process is thought to be the mode of origin for a salt dome itself.